The NZX is holding firm on its decision not to cancel Lyttelton Port Company (LPC) trades made while some shareholders were unaware a takeover offer was being announced.

The NZX said that because of significant public interest in the matter it was providing an update of its review of LPC share trades last Friday in the period between the release of a substantial security-holder (SSH) notice and an LPC trading halt.

Shareholder Mike Daniel is angry that about a dozen trades in LPC shares last Friday have not been cancelled or reversed.

A takeover offer was announced on the NZX, but Daniel said a trading halt was not called soon enough, with the offer initially at the bottom of the SSH.

The NZX said it could not cancel the trades as it had limited powers under NZX participant rules.

Some brokers, who did not want to be named, said the NZX was wrong not to fix what was obviously a mistake.

The NZX said today that its commitment to a transparent market included the principle that all price-sensitive information in relation to listed issuers was available to all investors at the same time.

On the LPC trades, all market participants had the same information access, the exchange said.

LPC was not aware of the intended takeover before the SSH was received from Port Otago.

Port Otago, a 15.48 per cent shareholder in LPC, gave details that it would sell its LPC shares to Christchurch City Holdings Ltd (CCHL), an investment arm of the city council under a lockup agreement.

CCHL plans to launch a full takeover offer this week at $3.95 a share.

It will pay a 20-cent dividend, taking the total payout to existing shareholders to $4.15 a share.

The two parties that entered into a lockup agreement, CCHL and Port Otago, were not listed issuers and therefore had no obligation under NZX rules to release material information beyond the SSH filings, the exchange said.

"NZX does not intend to take any action to cancel or reverse the trades," it said.

"The trades were not made in error; ie, they were not erroneous."

The NZX's investigation was continuing, and further action may be taken if evidence of a breach of NZX rules was identified.

The NZX was reviewing its process in relation to the release of SSH notices.

"We are also working with the Financial Markets Authority to ensure other substantial shareholders and companies that are not regulated by NZX are made aware of their need to contribute to an orderly and transparent market," it said.

Substantial shareholders - those who hold more than 5 per cent of a listed issuer - have a requirement to inform the NZX and the listed issuer concerned immediately of any shareholding change.